1. Field of the Invention
The present invention generally relates to a circuit substrate and a method of fabricating the same, in particular, to a circuit substrate in which a solder mask covering a surface thereof has at least one opening, with an upper diameter larger than a lower diameter, for exposing a bonding pad, and a method of fabricating the same.
2. Description of Related Art
Recently, with the increasingly high demand of the market on electronic products and the advanced processing technique, more and more 3C products emphasize portable convenience and popularization of the demand of the market, the conventional signal chip packaging technique cannot meet the increasingly new demand of the market, it has become a well-known product trend to design and manufacture products with characteristics of being light, thin, short, and small, an increased packaging density, and a low cost. Therefore, under the precondition of being light, thin, short, and small, various integrated circuits (ICs) with different functions are integrated by using various stacking packaging manners, so as to reduce the package volume and package thickness, which is a main stream for the research on the market of various packaging products. As for the current various packaging products under mass production, package on package (POP) and package in package (PIP) products are new products as the main stream of the research in response to the development trend.
FIG. 1A is a schematic cross-sectional view of a conventional POP package structure, in which pre-solders on a circuit substrate are not bonded with solder balls on an opposite package, and FIG. 1B is a schematic partially-enlarged view after the pre-solders as shown in FIG. 1A are bonded with the solder balls. Referring to FIGS. 1A and 1B, when the first package 100 and the second package 200 are stacked, through reflow soldering, solder balls 110 disposed on the first package 100 are turned into a melted state, and then bonded with corresponding solder balls 210 on the second package 200, thereby being electrically connected to each other.
However, as shown in FIG. 1B, during the reflow soldering process, since an opening at an upper end of the solder ball 110 is relatively small, and due to the surface tension, after the solder balls 110 are melted, they may be spilled upwards via the opening at the upper end, and a lower part thereof may be turned in a hollow state. As a result, the first package 100 and the second package 200 cannot be electrically connected to each other, such that the yield of the produces is reduced.